Fused Quartz Helix Sensors As shown in Figure 5.4i, the process pressure causes the free end of the helix to rotate together with the mirror attached to it. The quartz helix is mounted above a large precision gear that can rotate concentrically around the pressure sensor. A lamp and a pair of photo cells are attached to the gear. The light from the lamp is reflected from the mirror and distributed equally on the two photo cells. When the process pressure changes, the free end of the helix rotates the mirror, unbalancing the photo cells. This signal causes the balancing motor to turn the gear until the light falling on the photo cells is equally distributed once more. Consequently, the gear follows the movement of the free end of the helix without mechanical contact. The rotation of the gear is registered by a digital counter that is calibrated in pressure units.